The use of machines for driving elements in the ground has widespread applications in the formation of foundations for structures of all types and the elements which can be driven into the ground may vary in shape depending upon the particular purpose.
Tube piles, for example, which can be filled with concrete once driven in the ground, represent a major kind of pilings in widespread use. Sheet piling, for coffer dams or merely to enclose a particular region or as a barrier, are also common, the sheets being driven into the ground in interlocked contiguous relationship. Solid-cross section or post-type piles may also be driven into the ground.
Pile drivers which have been provided heretofore for this purpose are usually in one of three categories:
Electric or hydraulic vibrators operating at high frequency and which vibrate the piles into the ground.
Pneumatic hammers or pile drivers operating at an intermediate frequency and in which a reciprocatable mass is driven in alternating directions by compressed air.
Falling mass pile drivers as diesel, pneumatic, steam or hydraulic actuation which function at low frequencies. Generally an expanding or fluid pressure serves to raise the mass and is vented to permit the mass to fall at least in part by the action of gravity upon the pile or an anvil acting thereon.
The electric or hydraulic vibrators generate sinusoidal unidirectional force by coupling a rotary eccentric mass to the element to be driven into the ground. These machines generally operate effectively in sandy soils but in clay or rocky terrains give relatively poor results. Pneumatic hammers utilize a piston which is driven upwardly and downwardly alternately by expansion of the compressed air have been found to give better results in clay soils but the properties of these machines are a compromise between those with the vibrating machines described above and the falling-mass machines. The range of intermediate frequencies of the shocks or impacts delivered to the element usually between 2 and 10 Hz, precludes the use of large masses for impact transfer. Consequently, the amount of energy which can be transferred at each impact to the element may not be sufficient for certain types of elements.
In the falling mass machines, a relatively heavy piston, constituting the energy-transfer mass, is lifted by a fluid such as air, steam, oil under pressure, or another gas, e.g. a gaseous mixture which is exploded to provide an expansion (diesel) and then drops in free fall on the element to be driven into the ground.
Because of the large mass which is used and the nature of the mechanism for lifting same, this machine cannot function at an intermediate or high frequency and generally the frequency of operation of a falling mass unit is of the order of 1 Hz. These machines are effective in all types of terrain except possibly for extremely porous or loose soil utilizing diesel drop hammer machines.
In the falling-mass drop hammers, better results may be obtained in the lifting performance with one of the three forms of energy used, i.e. diesel, pneumatic and hydraulic, than in others while in other systems, the free-fall performance or reaction force relationships may give better results. However, these earlier systems are not fully effective in all cases and thus it has been desirable to seek alternative systems for driving piles or the like into the ground.